1. Field
The present application relates to AC power control, and, more particularly, to digital control of AC impedance.
2. Description of the Related Art
With the proliferation of electronic systems based on submicron feature transistors, power quality has become a major concern for end users as well as the power generation and distribution authorities in the world. Voltage sags and surges are a very common phenomenon in many distribution circuits, particularly in overloaded distribution systems. Voltage sags and surges seem to be non-uniformly distributed and tend to follow the daily loading patterns of the utility. In many developing countries these problems may be more amplified. For example, FIG. 1 indicates a typical pattern of voltage fluctuations in an urban location sub-circuit in Sri Lanka.
In tackling the power quality issues on a given equipment site, three major considerations should be voltage sags and surges, transient surges such as lightning or power system originated ones, and harmonics and flicker, etc. In tackling voltage sags and surges at a given site, an AC voltage regulator is a particularly useful power conditioning equipment. Common regulator techniques used in AC voltage regulators are: (i) motor driven variacs; (ii) transformer tap changers; (iii) ferro-resonant regulators; (iv) thyristor-based systems; and (v) solid state AC regulators. Table 1 summarizes the performance of these regulator techniques from a practical standpoint. The information below is applicable to the single phase systems and output ratings from few 100 watts to few kilowatts.
TABLE 1Comparison of AC voltage regulatorsFamilyTechniquesAdvantagesDisadvantagesMotor driven variesA servo motor-basedSimple constructionBulkyauto transformer withHigh capacitySlow responsea voltage feedbackSimple electronicsCan get stuck at theloopHigh efficiencylowest input voltageand create an over-voltage when the linevoltage goes normalTransformer tapA transformer withHigh efficiencyIf input voltagechangersmultiple taps and aEasy to designfluctuates frequentlyfeedback loop is usedSimple construction“tap dancing” couldto automaticallyLow costoccurchange the taps at theArcing in taps canoutput side when thecreate problems withinput voltageinductive loadsfluctuatesVoltage transientscan appear at outputduring tap changesThyristor-basedA series secondaryCompactHigh harmonicdesignswinding or an autoLow costcontent at the outputtransformer is usedEfficientCould causewith a thyristor phaseFast responseproblems withcontrolled techniqueinductive loadsat output to maintainFiltering at outputthe RMS voltagemay be necessary forconstantreducing RFI/EMIissues on the load sideFerro-resonantA precisely gappedVery reliableNon sinusoidalregulatorstransformer is used inSimple designoutput with flattenedresonance with awithout muchtopcapacitor to create aelectronic circuitsRegulation dependsresonant circuit, whileCan withstand aon the load powercore saturation is usedfractional or few cyclefactorfor regulating theoutage at the inputExtremely sensitiveoutput sidesideto frequencyDifferential modefluctuations at thetransients can beinput side (such as intoleratedcases of smallgenerator is used assupply)Low efficiency andno load powerconsumption of 20-30%of the VA ratingSolid state typesEither linearWide input range isComplex circuitryamplifier-basedpossible(In switchingtechnique orCompact designtechnique-basedswitching technique-may be possible (withversions)based compensation isa switching techniqueRFI/EMI problemsusedfor voltage buck orin (switchingboost)technique-based ones)Reliability issues inenvironments withhigh common modetransient surges)
Given the variety of techniques described above used in commercial circumstances in the last two decades, a consistently popular technique has been the ferro-resonant-based techniques. Invented by Joseph Sola in 1930s, this technique is popular due to its operational reliability, simple construction, and the ability to regulate a fraction to a couple of AC cycles due to its LC resonant circuit tuned to the line frequency.
However, this technique is not energy efficient and dissipates approximately 200-300 watts per each KVA of its output due to the saturation behavior of the gapped transformer operating near saturation. In addition, output regulation of these systems can be very dependant on the power factor of the connected load. Other serious issues with these “ferrel” versions include, when the UPS or power conditioner is powered by a low cost standby generator and the frequency is not exactly 50 or 60 Hz, and with fluctuations with the loading on the generator, the resonant circuit can mistune and create various malfunctions. In Sri Lanka, for example, during the long power outage times due to drought conditions, some commercial line interactive type UPS systems with ferro-based AC regulation have ended up in failures when powered by low cost standby generators.
Apart from the above approaches used in commercial AVR techniques, there are a few other published approaches to achieve AC voltage regulation in single phase power conditioners. Many of these are based on a series AC voltage component generated by a switching pulse width modulation (PWM) scheme, or an electronic transformer-based technique (which also is based on a PWM switching technique). Another variation of a PWM-based series connected AC regulator technique is described in Tsai, M. T.; “Analysis and design of a cost-effective series connected AC voltage regulator”, Electric Power Applications, IEEE Proceedings, Volume 151, Issue 1, 9 Jan. 2004 Page(s): 107-115, and Tsai, M. T.; “Design of a compact series-connected AC Voltage regulator with an improved control algorithm”, Industrial Electronics, IEEE Transactions on, Volume 51, Issue 4, August 2004 Page(s): 933-936, each of which is herein incorporated by reference for all purposes. Almost all of these techniques have their own advantages and disadvantages. One common problem is the RFI/EMI issues due to PWM switching schemes. In particular, the complex issues of adapting them to medium and high output orders of few 100 W to few kilowatts in single phase power conditioning environments.
Thus, a continuously variable AC resistance may be used as the primary control element and provides advantages such as fast response, lower distortion, less dependence on load power factor and easy to manufacture magnetic components compared to a system based on a ferro-resonant transformer. Similarly, this design approach could be effectively used in an AC electronic load suitable for UPS and power conditioner testing, etc., if the non linear distortion of current in the power transistor array can be minimized by a suitable processor control approach.
Therefore, developing the basis of a digital control algorithm for reducing the harmonic distortion of the current waveform for providing better or comparable specifications with ferro-resonant types, with a simple and easy to build transformer, is desired. Embodiments could also be used as the basis for an AC electronic load suitable for AC line voltage requirements.